1. Field of the Invention
The present invention generally relates to a method of making an extruder sleeve, and, more particularly, to a method for making a dual straight-hole sleeve which has a bimetallic inner bushing and two outer sleeve parts.
2. Description of Related Art
A conventional extruder sleeve with a dual straight-hole design is shown in FIGS. 1A and 1B. It is integrally made as a single piece, with an inner surface having a hardened layer of 0.3-0.6 mm treated by nitridation. Because the hardened layer is only 0.3-0.6 mm, it is easily worn out by the extruded plastic material in the extruder. Therefore, conventional extruder sleeves have inadequate wear resistance and low durability.